This invention relates in general to new and improved vibration dampers for protecting suspended elongate members against vibration-induced fatigue failures adjacent their points of support or suspension. The invention is more particularly addressed to an improved Stockbridge type damper which has the capability of withstanding undesirable strains resulting from ice buildup on the conductor.
It is a well-recognized and accepted fact that suspended elongate members are vibrated by wind blowing thereagainst and that aeolian vibrations produced therein are a frequent source of fatigue failures in the elongate members adjacent their points of support or suspension. In the case of stranded cables, failure by fatigue may involve fracture of all, or less than all, of the strands incorporated in their fabrication. The aeolian vibrations complained of exist almost entirely in a substantially vertical plane, are particularly critical in the range of wind velocities from 2 to 15 miles per hour, and are caused by alternate formation of eddy wind currents or vortices moving above and below the longitudinal extent of an elongate member on the leeward side thereof.
The amplitudes of individual aeolian vibrations have been determined and observed to be relatively small and normally less than the diameters of the elongate members so vibrated. In addition, it has been observed and determined that there are many loops and node points in any given length of span of a vibrating elongate member, with 100 loops in a 1000-foot span of a suspended electrical transmission line being a common occurrence.
The aeolian vibrations herein described are in contrast to the less frequently encountered and observed phenomenon of "galloping", which characterizes substantial and perceptibly large amplitudes of displacement of suspended elongate members, such as one or two loops per span, experienced with suspended electrical transmission cables and similar elongate members under wind and icing conditions.
The loop length, frequency and amplitude of aeolian vibrations vary over wide ranges of values for any given length of span, cross-sectional area of elongate member and tension within the suspended member, in accordance with the direction and velocity of the wind inducing the complained of vibrations. Manifestly, energy-absorbing devices, termed vibration dampers, have been devised and are in regular use in attachment on suspended elongate members, subject to aeolian vibration, for the express purpose of protecting the elongate members against fatigue failure adjacent their points of suspension.
One of the most effective and widely used dampers for preventing and/or minimizing aeolian vibrations in suspended elongate members is known commercially as the Stockbridge damper. This damper, as conventionally fabricated and used for about 50 years, is described and illustrated in U.S. Pat. No. 1,992,538, issued Feb. 26, 1935. In its essentials, the conventional Stockbridge damper herein referred to comprises a pair of inertia members, or weights, separated by an axially extending resilient member to the opposite ends of which one each of the inertia members is secured. A clamp secured to the resilient member, intermediate the inertia members, is adapted to attach the damper to a suspended elongate member to be protected thereby and, in operation, aeolian vibrations in the suspended elongate member are transmitted to the damper to shake or vibrate the inertia members under the energy-absorbing restraint of the resilient member supporting the same.
While the Stockbridge damper works well under normal weather conditions, under icy conditions its failure rate increases dramatically. Ice forms on the conductor to create more surface area for wind contact. As a result, wind which normally is easily handled by the damper in ice-free weather now produces aeolian vibrations in the conductor that exceed the damper's damping capacity. These vibrations cause the damper's inertia weights to pass through amplitudes that are beyond the endurance limits of the damper's resilient members, thereby resulting in resilient member fatigue failure in time. Trying to overcome this problem by employing larger dampers with greater damping capacity introduces even more trouble since such dampers in ice-free situations entrap aeolian vibrations at the damper clamp location, which thereby causes fatigue failure at this point.